There is a need for precision measuring systems which can be used to accurately determine positioning of an object. These measuring systems are particularly useful in calibrating and certifying machine tools, and for monitoring and controlling robots in industrial applications.
Five-axis precision laser measuring systems have been developed, such as the inventor's 5-D laser tracking system disclosed in U.S. Pat. No. 4,714,339. This system detects positioning in five "dimensions": X, Y, and Z displacement, pitch, and yaw. Laser detection of roll orientation has posed a more difficult problem, since roll does not necessarily result in any change in distance between the observing position and portions of the rolling element.
U.S. Pat. No. 5,064,289 to Bockman discloses a laser interferometer system which detects roll using a laser and two photodetectors. Bockman, however, uses two separated beam paths and a mirror assembly to measure roll.
U.S. Pat. No. 5,056,921 to Chaney and U.S. Pat. No. 5,363,196 to Cameron are similar to the Bockman patent in that they show roll measuring systems that use separated beam paths to measure roll. U.S. Pat. No. 4,804,270 to Miller et al. uses a diffraction grating to provide multiple beam portions to detect roll.
U.S. Pat. No. 5,418,611 to Huang et al. discloses a pitch and yaw measurement device which uses a polarizing beam splitter, two critical angle prisms, and two photodiodes to detect angular changes. However, this device does not provide roll detection.
A system was developed at the University of Michigan which uses two off-center retroreflectors on a common sensor head, aligned with two off-center laser beams, to detect roll. This general technique is relative inaccurate, and thus impractical for precision machine tool applications.
The inventor's company has sponsored research in the field of laser roll detection in collaboration with the University of North Carolina-Charlotte. As a result of this research, a laser roll detector was developed which uses a modulated, polarized laser beam which is passed through a Faraday cell, then a Glan-Thompson prism, and then to an intensity photodetector. The beam is dithered with a modulation frequency (e.g. 17 hz), and the dither frequency is then used to filter the intensity output to provide an output substantially free from the effects of beam intensity variations.
U.S. Pat. No. 3,915,572 to Orloff shows a laser Doppler velocimeter where two photodetectors have outputs connected to a differential amplifier to cancel disturbances common to the measured light beams. However, there is no suggestion that this technique could be useful in a polarized laser beam for roll detection.
None of these previous developments has provided an entirely satisfactory precision laser roll detector, and there is a need for an improved precision laser roll detector which can be used alone or as a component of a multiple axis position detecting system. Further, none of these techniques has provided a completely satisfactory 5-D detection system.